Why Do Slice Images Offer Higher Contrast?

Conventional film radiography offered a valuable non-invasive means of diagnosis for many decades. Its limitations were and are severe, however we are increasingly less aware of these due to the availability of alternative procedures. Imaging the brain by radiographs, for example, yields insufficient result in most cases. Roentgenologist tried to solve this dilemma by various measures, for example by using contrast media such as air in pneumoencephalography. In spite of the high effort and the great discomfort to patient these procedures only yielded very limited additional information. Computed tomography for first time offered the possibility to image brain structures in diagnostic quality with high contrast. How is this achieved? Several radiogical books offered the explanation that the high contrast of CT images is due to higher dose. I think this explanation is wrong. Although both radiography and CT use X rays, CT images display a quantity different from that in radiographs.

The principle of radiography is to record the radiation which is emitted from the focus of an x ray tube and attenuated by object to be examined with a detector, traditionally by film. This a conventional radiograph present the modulated distribution of radiation intensity and always offers a superposition image: all structure along the rays forms x ray focus to detector, i.e all volume element passed by any ray, contribute display the sum of all contribution to attenuation or,, mathematically speaking, the integral of attenuation component along time. We will return to these “line integrals” again when discussing the principle of computed tomography in next post.